The capture, transfer and use of geothermal energy has intrigued engineers around the world. Geothermal energy is emitted from deep within the earth and from radioactively decaying formations around the world, yet only in a few places is the heat level high enough to allow useful conversion to mechanical energy and/or electricity to occur.
Efforts to recover geothermal energy have included pumping fluids into hot rock formations through one well and up an adjacent well to a thermal recovery unit that transfers the heat energy to a separate power cycle. In very high temperature areas, such as California's geothermal valley where the majority of electricity produced by geothermal is located, water is injected into high-temperature formations and converted into high-pressure steam down-hole, which returns to the surface and is expanded through conventional steam turbines and condensed and exhausted out. Closed-cycle loops of heat transfer fluid are circulating in well bores for both cooling fluid, in the case of geothermal air-conditioning, or heating the fluid for use on the surface in either heating a facility or powering a low working temperature thermal engine to produce electricity.
Using the traditional non-phase change thermal fluids as heat exchange fluids requires a large volume of fluid to be pumped up and down to the formation level. Allowing formation water(s) that have been pumped through an underground formation to flow to the surface also brings along a variety of toxic and caustic chemicals. The nature of underground water has limited the ability to utilize enormous geothermal heat resources.
Geothermal heat resources are uneconomic given the current state of technology. One of the inherent laws of thermodynamics is that the difference in temperature between the heat source and the heat sink limit the efficiency of any thermal device (Carnot). Most of the earth does not have high-temperature geothermal energy available at a reasonable depth. In particular, most oil fields, because of the water-bearing strata and type of rock, do not possess high bottom hole temperatures.
Although water-bearing formations behind existing well pipe walls have high thermal conductivity and should be able to release heat for decades without temperature decay, the current technology cannot convert this heat to electricity profitably. What is needed is a way to capture, transport and release this heat energy at a higher temperature than the formation offers. This process must be environmentally safe and capable of very long life span.